Expressway ramp traffic control system



A ril 21, 1970 Filed Feb. 21, 1967 J. L. BARKER ETAL 3,508,191

EXPRESSWAY RAMP TRAFFIC CONTROL SYSTEM 5 Sheets-Sheet l SIGNAL SEQUENCETIMER AND CONTROLLER MONiTOR MONITOR LEVEL MONITOR INVENTORS JOHN L.BARKER CHARLES L. DUVIVIER LUDWIG R. P AT ATTORNEY April 21, 1970- FiledFeb. 21, 1967 J. L. BARKER ET AL EXPRESSWAY RAMP TRAFFIC CONTROL SYSTEM5 Sheets-sheaf 2 z 3 .J l w J E 5 5 Q 5 L .7 .7 7 7 2 r' 4 r o m c S g T'II IT II I I II I I I II I I I II I I I II I K II D! (I g I g I E II EI 5 I I 5 II 5 I 2 2 2 E N I II 2 I d I Tl-l I 5 II I 5 I g; I 5, II 3 II I II I I I II I I I II I l I 3 II L I I L I .n/ 0 o c\ a m 2 g 2 &

INVENTORS I- JOHN L. BARKER E CHARLES L. DUVIVIER E g LUDWIG R. PALLATATTORNEY A ril 21, 1970 R J. L. BARKER L 3,

I EXPRESSW AY RAMP TRAFFIC CONTROL SYSTEM Filed Feb. 21. 1967 5Sheets-Sheet 3 +120 VDC 1.3 U I320 432d Fm N4 I I62 $200K 1 3 IOOKCHARLES L. DUVIVIER LUDWIG R. PALLAT ATTORNEY April 21, 1970 J, L.BARKER ETAL 3,508,191

EXPRESSWAY RAMP TRAFFIC CONTROL SYSTEM Filed Feb. 21. 1967 .5Sheets-Sheet 4 CONTROLLED 334 MULTIVIBRATOR I NVENTORS JOHN L. BARKERFIG. 4 CHARLES L. DUVIVIER LUDWIG R. PA LAT BY M ATTORNEY United StatesPatent O US. Cl. 34036 12 Claims ABSTRACT OF THE DISCLOSURE A system forcontrolling entrance of vehicles onto a limited-access roadway from anentrance ramp. Gaps between vehicles on the roadway are detectedupstream of the ramp by means of vehicle detection pulses. Movement ofeach gap toward the ramp is matched by movement of a gap-indicatingsignal through a shift register. If a gap is long enough to permit avehicle to merge into it, the gap-indicating signal is transferred to asecond shift register which provides the required delay before operatinga trafiic control signal adjacent the ramp to release a vehicle,enabling it to reach the merging area when the gap is there.

BACKGROUND OF THE INVENTION This invention pertains to a highway trafficcontrol system. More particularly this invention pertains to a systemfor controlling the release of vehicles onto a limited-access roadwayfrom an entrance ramp leading to the limited-access roadway from anadjacent local street or frontage road.

Heavy traflic demands are placed upon the street systems of largemetropolitan areas during the periods in the morning and afternoon whenlarge numbers of commuters are traveling to and from work and duringother special occasions. This traffic frequently travels substantialdistances from the out-skirts of a large city to the downtown centralbusiness district, and, to enable it to travel more rapidly, numerouscities have developed systems of limited-access roadways, frequentlyreferred to as expressways, freeways, or parkways. These expressways arehigh cost facitities designed for heavy traflic flow at relatively highspeeds. During the peak trafiic periods, when the commuter traffic isheaviest, even these expressways become congested due to the largeamount of trafiic using them, breaking down the free flow of traflic.

Limited-access roadways generally pass under or over the city streetswhich make up the remainder of an arterial street system. Trafficseeking to enter an expressway is routed onto it via an entrance rampfrom an adjacent frontage road which may be an arterial street of thesystem. The expressway trafiic is generally moving at a relatively highspeed, for example in the range of 50 to 60 miles per hour. When thistrafiic is heavy, a vehicle attempting to enter it frequently 'must stopuntil its driver observes an opening or gap in the expressway trafficinto which he can merge. To be able to adequately observe the expresswaytraffic, the driver of the first vehicle waiting to merge usually findsit necessary to wait near the end of the entrance ramp, adjacent themerging area of the entrance ramp and the expressway. Since the mergingvehicle must start from a stand-still, it has to accelerate rapidly tothe speed of the expressway traffic. Consequently, a substantial gap inthe expressway traffic is necessary to enable the entering vehicle toreach the required speed and to merge safely. When the expresswaytrafiic is heavy, it is frequently necessary for the first vehicle onthe entrance ramp to wait for a long period of ice time before asuitable gap in the expressway trafiic is detected by the driver. As aconsequence, a line of vehicles frequently accumulates behind the firstvehicle, and this line often grows to such a length that it stretchesinto the adjacent frontage road and adds to the congestion on it and onthe remainder of the arterial street system. The results are thatcongestion on the expressway and on arterial streets is increased andthat drivers seeking to travel faster by means of the expressway mustspend considerable time waiting to enter the expressway. Therefore thedesired result of the expressway, namely to reduce overall travel time,is not achieved.

Attempts have been made to improve the traffic congestion at expresswayentrance ramps by closing selected entrance ramps during certain periodsof peak traflic congestion. While this somewhat improves the trafficflow on the expressway itself and avoids lines of waiting vehicles onthe local streets adjacent the expressway entrance ramp, it divertsvehicles which otherwise would utilize the expressway onto the arterialstreets of the local street system. As a consequence, these vehiclesrequire greater travel time because they are unable to utilize theexpressway, and the trafiic on the local arterial streets is increasedwith a resulting increase in congestion there.

Other attempts to improve trafiic flow at expressway entrance ramps haveincluded the use of traffic control personnel to signal waiting driverswhen to enter the expressway. This simply replaces the judgment of thedriver with the judgment of the traflic control personnel. Errors injudgment by the controlling personnel or failure of the waiting driverto heed the signals of the control personnel can result either in anacceptable gap in the expressway traflic being ignored because no driverenters it or in a collision of vehicles in the merging area of theentrance ramp and the expressway because a driver attempts to merge whenthere is no acceptable gap for him.

SUMMARY OF THE INVENTION In one aspect, the present invention is asystem for controlling the release of vehicles onto a limited-accessroadway from an entrance ramp which terminates in a merging area withthe roadway. The system is adopted for use with vehicle sensing means ofthe type which provides an indication of vehicle presence and speed andfor use with a signalling means which controls release on the entranceramp of vehicles heading toward the merging area. The signalling meanscan be located adjacent the entrance ramp a distance from the mergingarea, and so the vehicle can accelerate some distance before reachingthe merging area. This allows the vehicle to merge into a smaller gapthan is possible when the vehicle commences its acceleration in themerging area. The system includes means to detect an acceptable gap inthe expressway trafiic, and means to control the signal to release awaiting vehicle to enable it to accelerate and reach the merging area atthe same time the detected gap is at the merging area.

Another aspect of the invention includes means for inhibiting therelease signalling to any additional vehicle on the entrance ramp when apreviously released vehicle is stopped in the merging area because itsdriver has failed to merge into the preceding gap.

A further aspect of the invention includes means for signalling awaiting vehicle to enter the merging area even in the absence of anacceptable gap when the line of cars waiting on the entrance ramp hasreached such a great length that it is liable to interfere with thetrafiic on the frontage road.

An additional aspect of the invention includes means to signalcontinuously for waiting vehicles on the entrance ramp to enter themerging area when there is an exceptionally long gap in the expresswaytrafiic or when the expressway trafiic is so light that metering of theentering traffic is not required.

A still further aspect of the invention includes means for varying theminimum required acceptable gap length in accordance with acharacteristic of the trafiic flow on either the entrance ramp or thelimited-access roadway.

Yet another aspect of the invention includes means for controlling thetime-delay projection of the gap from the point at which it is detectedupstream of the merging area to the merging area in accordance with acharacteristic of the traffic flow on the roadway.

It is accordingly an object of the present invention to provide animproved system for controlling the merging of traffic onto alimited-access roadway from an entrance ramp.

It is another object of the present invention to provide an improvedlimited-access-roadway-entrance-ramp control system capable of detectinggaps in the roadway traffic into which a vehicle can fit and ofsignalling to cars waiting to enter the roadway that an acceptable gapis available.

It is a further object of the present invention to provide a controlsystem to detect a gap in the traffic on a limitedaccess roadway intowhich a vehicle can merge from an entrance ramp downstream from thepoint on the road- Way at which the gap is detected and to signal avehicle waiting on the entrance ramp at a time which will allow thevehicle to accelerate along the entrance ramp to reach the merging areaat the junction of the entrance ramp and the roadway when the gap hasreached the merging area.

It is yet another object of the present invention to provide such acontrol system for a limited-access roadway entrance ramp which insuresreduction of traffic congestion by overriding the regular operation ofthe control system during times of light trafiic or of long gaps intraffic on the roadway and during times when the line of vehicles on theentrance ramp is so long that it might interfere with normal trafficflow on the frontage road.

These and other objects and advantages will be apparent in the presentinvention from the following detailed description and claims,particularly when taken in conjunction with the accompanying drawings inwhich like parts are designated by like numerals.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a diagram depicting anentrance ramp leading from a frontage road to a limited-access roadwayand depicting in block form a prefered embodiment of the presentinvention.

FIGURE 2 is a representation, partially in schematic and partially inblock form, of a voltage level monitor suitable for use in theembodiment of FIGURE 1.

FIGURE 3 is a schematic diagram of a gap detector suitable for use inthe embodiment of FIGURE 1.

FIGURE 4 is a block diagram of a gap projector suitable for use in theembodiment of FIGURE 1.

FIGURE 5 is a block diagram of a signal sequence timer and controllersuitable for use in the embodiment of FIGURE 1.

DESCRIPTION OF A PREFERRED EMBODIMENT FIGURE 1 depicts a roadway formingone side of a limited-access divided roadway, the other side being onthe far side of the median strip 12 which is beyond the edge 14 ofroadway 10. Traffic on roadway 10 moves to the right, as shown by thearrow, (where the drive-tothe-right rule prevails), while traffic in theopposite direction travels on the other side (not shown) of the dualroadway. Each side of the divided roadway has several lanes for travel,and roadway 10 is shown by way of example in FIGURE 1 as having threelanes of travel.

Entrance ramp 16 connects the adjacent frontage road 18 to roadway 10,and cars traveling on frontage road 18 are able to enter roadway 10 bymeans of entrance ramp 16, as indicated by the arrow on ramp 16. By wayof example, frontage road 18 might be a part of the arterial streetsystem in a large urban area, while limited-access roadway 10 might bepart of an expressway, freeway, or parkway through or approaching theurban area. While frontage road 18 is shown parallel to roadway 10 inFIG- URE 1, it might run at an angle to roadway 10, passing over orunder it by "means of a bridge or underpass.

A merging area 20 is provided at the end of entrance ramp 16, where itjoins roadway 10. This merging area 20 enables cars entering the roadway10 from the entrance ramp 16 to travel for a short time parallel toroadway 10 before actually entering its right-hand lane. As depicted inFIGURE 1, frontage road 18 is provided for one-way travel to the right,as shown by the arrow, and a second lane 22 is provided on frontage road18-, immediately before the entrance ramp 16, for the convenience ofvehicles turning onto entrance ramp 16 from the frontage road 18, butsuch a turning lane might not be provided, and frontage road 18 could bea two-way street.

A control signal 24 is mounted adjacent entrance ramp 16, at a pointabout half-way between frontage road 18 and merging area 20, forexample. Control signal 24 might be a standard red-yellow-green trafficlight, for example. Alternatively, it might be any other type of trafficcontrol signal or device, such as a gate which rises and falls acrossentrance ramp 16.

A number of vehicle sensing means or vehicle detectors monitor thetraffic flow on roadway 10, entrance ramp 16, merging area 20, andturning area 22. As depicted in FIGURE 1, detector D1 is located at adistance upstream from merging area 20 to monitor the traffic in therighthand lane of limited-access roadway 10 as the traflic approachesmerging area 20 on roadway 10. Detector D1 is mounted far enoughupstream from merging area 20 to enable use of the output informationobtained from it, as is explained hereinafter.

Detector D1 indicates the passage of each vehicle through its detectionzone, and it indicates the speed of vehicles as they pass through itsdetection zone. Detector D1 may comprise separate detectors to performthese two functions, or they may be performed by the same detector. Thedetection zone of detector D1 is depicted in FIGURE 1 by the outline D1,and as shown there it includes a brief stretch of the right-hand lane ofroadway 10. Detector D1 might be any one of several types which arecapable of providing the required signals. For example, it could be asonic detector, such as that disclosed and claimed in co-pendingapplication Ser. No. 551,692 by Bernard J. Midlock, entitled SonicVehicle Detector, filed May 20, 1966, now US. Patent 3,362,009 issuedJan. 2, 1968, and assigned to the same assignee as the presentinvention. Alternatively, detector D1 could be an inductive loopdetector, such as a wire loop and a Model LD-l Loop Vehicle Detectormanufactured by Automatic Signal Division of Laboratory for Electronics,Incorporated, and described in that companys LD1 Loop Detector BulletinD-168, copyright 1966 by Laboratory for Electronics, Incorporated, byway of example.

Detector D2 is mounted in a position to monitor vehicles as they passthrough merging area 20. When a vehicle passes through the detectionzone of detector D2 (indicated by outline D2 in FIGURE 1) the detectorprovides an output indicative of the speed of that vehicle, and when avehicle stops in the detection zone of detector D2, the detectorprovides a continuous output indicative of the presence of that vehicle.Detector D2 could be any one of several types capable of providing therequired signals. By way of examples, it could be either the sonicdetector or the inductive loop detector described with reference todetector D1. To insure coverage of the entire merging area 20, two orthree detectors might be used in parallel.

Detector D3 is mounted in a position to monitor vehicles on entranceramp 16 after they have passed signal 24. Detector D3 is required togive outputs indicative of the passage of vehicles. It need not providevehicle presence or vehicle speed information; it only is required toprovide an indication each time a vehicle passes through its detectionzone (shown in FIGURE 1 as outline D3). Detector D3 might be any ofseveral types, for example, it could be a sonic detector or an inductiveloop detector, as described above with reference to detector D1.Alternatively, it could be a pressure sensitive vehicle detectorembedded in the surface of entrance ramp 16. One such pressure sensitivevehicle detector is the Model HR manufactured by Automatic SignalDivision of Laboratory for Electronics, Incorporated, and described inthat companys Model HR and HRD Pressure-Sensitive Vehicle DetectorsBulletin D-161, copyright 1957 by Eastern Industries, Incorporated,predecessor in title to Laboratory for Electronics, Incorporated.

Detector D4 is mounted in a position to monitor vehicles on entranceramp 16 in front of signal 24. Detector D5 is mounted in a position tomonitor vehicles in turning lane 22. Detectors D4 and D5 are required toprovide indications of the presence of vehicles within their respectivedetection zones (shown by outlines D4 and D5 respectively in FIGURE 1);but they need not provide vehicle speed information. By way of example,detectors D4 and D5 could each be either a sonic detector or aninductive loop detector, as described above with reference to detectorD1. To insure coverage of the entire turning lane 22, two or threedetectors might be used in parallel for detector D5.

Detectors D1 through D5 and control signal 24 are connected to thetraflic control circuitry, which might be located in a control boxadjacent the entrance ramp 16 or alternatively which might be locatedremote from entrance ramp 16, by way of examples.

Detector D1 is connected to the input of speed computer 30 and providesto the speed computer 30 signals which are indicative of the speed ofvehicles passing detector D1 in the right-hand lane of limited-accessroadway 10. Speed computer 30 develops as its output a voltage which isindicative of the speed of vehicles passing detector D1. This outputfrom speed computer 30 is applied via line 31 as an input to levelmonitor 32, which is capable of providing an output indication on anyone of a plurality of output lines, represented in FIGURE 1 by line 33.The particular output line from level monitor 32 which provides anoutput indication is dependent upon the level of the input voltageapplied to the level monitor 32 via line 31 from speed computer 30. Theoutput from speed computer 30 is also applied by line 31 as an input tolevel monitor 34 which provides an output when the input applied to itfrom speed computer 30 indicates that the speed of vehicles passingdetector D1 is above a preset level, as hereinafter described.

Detector D1 provides to volume computer 36 a signal indicative of thepassage of a vehicle past detector D1. Volume computer 36 provides anoutput voltage which is indicative of the volume of traific passingdetector D1. Thus, the voltage on the output of volume computer 36 isindicative of the number of vehicles per unit time passing detector D1.This output voltage from volume computer 36 is applied by line 37 as aninput to level monitor 38 which provides an output when the inputapplied to it on line 37 from volume computer 36 indicates that thevolume of trafiic passing detector D1 is below a pre-set level, ashereinafter described.

The outputs from level monitors 34 and 38 are applied as the two inputsof AND gate 40, which has its output connected via line 42 to signalsequence timer and controller 44. The output of controller 44 isconnected via line 46 to signal 24 to control the operation thereof.

Detector D1 is connected by line 47 to gap detector 48 to provide to thegap detector a signal indicative of the passage of a vehicle pastdetector D1. Thus, the time between receipt of signals on line 47 by gapdetector 48 is indicative of the spacing betwen consecutive vehicles inthe right-hand lane of limited access roadway 10. Gap detector 48measures the time spacing or gap between consecutive vehicles passingdetector D1, and when this gap reaches a pre-set minimum acceptable timelength, gap detector 48 provides an output on line 49 to gap projector50. This output indicates that there has been detected a gap of aduration sufficient for an entering vehicle from entrance ramp 16 tomerge into the traffic on the right-hand lane of roadway 10. After sucha minimum acceptable gap has been detected, gap detector 48 continues tomeasure the gap, and if the gap is great enough to permit additionalvehicles to merge onto roadway 10, gap detector 48 generates a secondggtput signal on line 51 which is passed to gap projector Line 47 fromdetector D1 is connected to gap projector 50 to provide to the gapprojector a signal indicative of the passage of a vehicle past detectorD1 on roadway 10. Gap projector 50 also receives as an input the signalon line 49 from gap detector 48 which indicates that there has beendetected in the traffic on roadway 10 a gap of a duration sufficient foran entering vehicle from entranceramp 16 to merge with that traffic. Inaddition, gap projector 50 is connected by lines 33 to the outputs oflevel monitor 32, which indicate the speed of tratfic on roadway 10.This speed is substantially identical with the speed at which thedetected gap is moving down roadway 10 from detector D1 toward mergingarea 20.

When the vehicle passage signal input on line 47 from detector D1 to gapprojector 50 indicates that the start of a gap has reached detector D1,gap projector 50 commences to project a gap-indicating electrical signalthrough a storage system to simulate or match the passage of the actualgap in trafficrflow down roadway 10 toward merging area 20. If the gapis too short for a vehicle from entrance ramp 16 to merge into thetraific on roadway 10, then no input is applied on line 49 to gapprojector 50 from gap detector 48, and so the next vehicle passagesignal applied on line 47 to gap projector 50 erases the gap-indicatingsignal. However, if gap detector 48 determines that the gap in traflicflow is long enough for an entering vehicle to merge, then before thegap-indicating signal is erased, gap detector 48 applies a signal online 49 to gap projector 50. This signal from gap detector 48 causes gapprojector 50 to generate an output at a time when gap projector 50predicts that the detected gap is passing a release point on roadway 10.The release point, shown in FIGURE 1 as release point R on roadway 10,is located at a distance upstream from merging area 20. Control signal24 signals to a vehicle waiting on entrance ram-p 16 to advance towardmerging area 20 from a point immediately in front of control signal 24at the time the detected gap is passing the release point on road-way10. As a result, the vehicle reaches merging area 20 and merges ontoroadway 10 with substantial speed at the same time the detected gap ispassing the merging area.

The output from gap detector 50, which occurs when the detected gap ispassing the release point on roadway 10, is applied by line 52 to thefirst signal input of IN- HIBITED-AND gate 53 which passes it to signalsequence timer and controller 44. If the detected gap is of great enoughlength for more than one vehicle to enter the traffic on roadway 10,then gap detector 48 applies an input on line 51 to gap projector 50,and the gap projector is enabled to generate an output on line 54 at thetime gap projector 50 predicts that the long duration gap is passing therelease point on roadway 10.

This second output from gap projector 50 is applied by line 54 as aninput to signal sequence timer and controller 44. Controller 44 thencauses control signal 24 to continuously signal to vehicles on entranceramp 16 to advance to merging area 20 so long as this long duration gapis passing the release point.

Vehicle detector D2 supplies to speed computer 56 an input indicative ofthe speed of cars passing through merging area 20. Speed computer 56generates an output voltage from this input and applies this outputvoltage by line 57 to level monitor 58. Level monitor 58 has a pluralityof output lines, indicated in FIGURE 1 by line 59. All of the outputlines 59 from level monitor 58 are connected to gap detector 48. Theparticular output line from level monitor 58 which supplies anindication to gap detector 48 at any given time is determined by thespeed of vehicles which have passed through the detection zone ofvehicle detector D2 just prior to that time. This information isinterpreted by gap detector 48 as indicative of the ease With which carsfrom entrance ramp 16 are merging into the traffic on roadway 10. Basedon this information, gap detector 48 adjusts the required duration ofthe gap in traffic on roadway which must be measured before outputs aresupplied on lines 49 and 51 from gap detector 48 to gap projector 50.

During the time that a vehicle is within the detection zone of vehicledetector D2, detector D2 supplies an input to timer 60. If this input totimer 60 is of a long duration, it indicates that a vehicle has stoppedin merging area 20, rather than entering a gap in the trafiic on roadway10. When this occurs, timer 60 generates an output which is applied tothe inhibit input of INHIBIT- -ED-AND gate 53 and to the inhibit inputof INHIBIT- ED-AND gate 62.

Vehicle detector D3, which is located in a position to detect vehicleson entrance ramp 16 just after they have passed control signal 24, isconnected by line 64 to controller 44.

Vehicle detector D4, which is located in a position to detect vehicleson entrance ramp 16 just in front of control signal 24, generates asignal when a vehicle is within its detection zone. This signal isapplied as an input to the second signal input of INHIBITED-AND gate 53and to the first signal input of INHIBITED-AND gate 62. INHIBITED-ANDgate 53 has its output connected by line 66 to signal sequence timer andcontroller 44.

Vehicle detector D5, which is located in a position to detect vehiclesin turning area 22 of frontage road 18, is connected to timer 68. Timer68 has its output connected to the second signal input of INHlBITED-ANDgate 62, the output of which is connected by line 70 to signal sequencetimer and controller 44. Vehicle detector D5 provides a signal to timer68 when vehicles are within its detection zone. A continuing signal fromdetector D5 to timer 68 occurs when a vehicle is stopped in turning area22. This indicates that the traffic on entrance ramp 16, Waiting to passcontrol signal 24, has formed a line which is approaching frontage road18. Once this signal from detector D5 has been applied to timer 58 for along enough time, the timer 68 generates its output which is applied toINHIBITED-AND gate 62.

Each of the components of the system depicted in FIGURE 1 may be ofstandard design. Volume computer 36 is required to generate an outputvoltage indicative of the volume of traffic flow past vehicle detectorD1. Numerous such volume computers are known in the art. By way ofexample, one volume computer suited for this application is disclosed inUS. Patent No. 3,059,232, issued Oct. 16, 1962, to I. L. Barker andassigned to the same assignee as the present invention. The volumecomputer there disclosed requires that with each vehicle passing vehicledetector D1 the input line to the volume computer be grounded. Thisrequirement is met by the sonic vehicle detector and by the inductiveloop vehicle detector described above with reference to vehicle detectorD1. 1

Similarly, speed computers 30 and 56 are required to provide outputvoltages indicative of the speed of vehicles passing vehicle detectorsD1 and D2 respectively. Suitable speed computers are known in the art.By way of example, one speed computer suitable for this application isdisclosed and claimed in the copending application entitled SpeedAveraging Circuit by Ludwig Pallat, one of the co-inventors hereof,filed the same date that the present application is filed and assignedto the same assignee as the present invention. The speed computer theredisclosed and claimed requires as an input a pulse having a durationwhich is inversely proportional to the speed of the passing vehicles.The sonic detector and the inductive loop detector described above withreference to vehicle detector D1 satisfy this requirement. As anotherexample, US. Patent 3,059,232 issued Oct. 16, 1962, to J. L. Barker, andassigned to the same assignee as the present invention, discloses aspeed averaging circuit suitable for use as speed computers 30 and 56when vehicle detectors D1 and D2 are Doppler radar detectors of the typedisclosed in that same patent.

Level Monitors-FIGURE 2 Each of the level monitors 32, 34, 38, and 58receives a voltage level as an input, and each generates an output onone of a plurality of output lines, with the particular output which isgenerated dependent upon the level or magnitude of the applied inputvoltage. A level monitor suitable for this use is depicted in FIGURE 2,although, of course, other level monitors might be utilized. The levelmonitor of FIGURE 2 is made up of a plurality of identical sections 78,designated 78a, 78b, 78c 7811. Only the first section 78a is shownschematically, but the remainder are identical. The number of sectionsprovided for a particular level monitor depends upon the number ofoutput levels into which the input voltage must be divided.

In each section 78, a source of positive potential, such as +15 voltsDC, is tied to the first end of the fixed resistance of potentiometer80, the second end of which is tied to ground. Similarly, a source ofnegative potential, such as 15 volts DC, is tied to the first end of thefixed resistance of potentiometer 82 which has its second end tied toground. The arms of potentiometers 80 and 82 are connected to the twoends of the fixed resistance of potentiometer 84. The arm ofpotentiometer 84 is coupled through resistor 86 to the base of NPNtransistor 88, which has its collector tied to the source of positivepotential and its emitter coupled through resistor 90 to the source ofnegative potential. The base of transistor 88 is also connected tothefirst end of resistor 92.

Input line 94 to the level monitor represents input line 31, 37 or 57 ofFIGURE 1 and is divided into lines 94a, 94b, 94c 94n, connectedrespectively the inputs of sections 78a, 78b, 78c 7811. Within eachsection 78, the second end of resistor 92 is connected to thecorresponding input line 94 to which the input voltage level is applied.

The emitter of transistor 88 is tied to the base of PNP transistor 96,which has its collector coupled through resistor 98 to the negativevoltage source and its emitter coupled through resistor 100 to thepositive voltage source. The emitter of transistor 96 is also coupledthrough resistor 104 to ground. The collector of transistor 96 is tiedto the base of PNP transistor 102 which has its emitter tied to ground.The collector of transistor 102 is connected to the cathode of diode106, the anode of which is tied to the source of negative potential.Capacitor 108 is coupled between the base and the emitter of transistor102 to filter out high frequency noise.

The collector of the transistor 102 in each section 78 is connected toone side of the coil of a corresponding relay 110a, 110b 11011, theother side of which is tied to the source of negative potential. Eachrelay 110 has contacts made up of moving contact or armature 112,normally-closed contact 14, and normally-open contact 116. Armature112a, associated with section 78a, is tied to ground. Normally-closedcontact 114a is connected to output line L1, which signifies that theoutput voltage applied to the level monitor is within the first levelrange. Normally-open contact 116a is tied to armature 112b of the nextsection 78b. Normally-closed contact 114b is tied to output line L2,which signifies that the input voltage is within the second level range.Normally-open contact 116b is tied to armature 1120 of section 780.Similarly, each normally-closed contact 11411 is tied to an output lineLN, and each normally-open contact 116 (ID-1) is tied to the armature112n of the next section. The last normally-open contact 11611 isconnected to an output line L(N+l). The output lines LlL(N+1) make upthe output lines from the level monitor, such as lines 33 and 59 (FIGURE1).

With no input applied to line 94, the emitter of each transistor 88 isslightly negative, and each transistor 96 is on. Therefore, eachtransistor 102 is off, and each relay coil 110 is isolated from ground.Since relay coil 110a is deenergized, armature 112a applies groundthrough normally-closed contact 114a to output line L1.

In each section 78, potentiometers 80, 82 and 84 determine the inputvoltage level required to cause energization of the corresponding relaycoil 110. By way of example, if a level monitor is required to provideoutputs indicative of speed over a speed range of zero mph. to 100 mph.in increments of ten m.p.h., then nine sections 78a through 781' areutilized. Each potentiometer 80 is set so that the voltage on its armrepresents zero mph, and each potentiometer 82 is set so that thevoltage on its arm represents 100 mph Then the arm of potentiometer 84ain section 78a is set to a voltage such that, when the input applied toline 94 is a voltage level representing m.p.h., transistor 88a conductsmore heavily, turning transistor 96a off and transistor 102a on. Thisenergizes relay coil 110a. Similarly, potentiometer 84b is adjusted sothat when the input applied to line 94 is a voltage level representing20 mph, relay coil 11012 is energized. The corresponding potentiometers840 through 84i are each adjusted so that the corresponding relay coils1100 through 1101 are energized at 30 mph, 40 mph, etc., through 90 mph.

Ground is applied to output line L1 when the input voltage on line 94indicates a speed in the range from zero mph. to 10 mph; ground isapplied to output line L2 when the input voltage indicates a speed from10 mph. to 20 mph, etc. For a speed between 80 mph. and 90 mph, groundis applied on output line L9, tied to normally-closed contact 1141'. Fora speed in excess of 90 mph, ground is applied on output line L10, tiedto normally-open contact 116:. This, of course, is only an illustrativeexample, and the number of sections 78 and the settings of thepotentiometers 84 are selected to divide the input voltage into therequired number of input level ranges and to provide indications onlines L1, etc., to meet the requirements of the particular application.While this illustrative example considers an input voltage indicative ofvehicle speed in miles per hour, other inputs can, of course, beprovided. Thus, such a level monitor can divide traffic volumeinformation into vehicle per hour ranges, for example.

Thus, for example, level monitor 32, which provides speed levelindications from speed computer 30 to gap projector 50*, might includenine stages having ten output lines, L1 through L10, as described in theabove illustrative example. Level monitor 34 requires only a singlestage 78 to provide an indication from speed computer 30 to AND gate 40when the speed of vehicles passing detector D1 is above a pre-setminimum level. In that single stage 78, the normally-closed relaycontact 114 is not connected to anything, while the normallyopen contact116 is tied to the output line, providing the signal from the levelmonitor 34 to AND gate 40. The potentiometer 84 is adjusted so that anoutput is provided from normally-open contact 116 when the speed ofvehicles in the right-hand lane of roadway 10 is above a pre-set minimumlevel.

Level monitor 38 provides a signal to AND gate 40 when the volumecomputer 36 output indicates that the volume of traffic in theright-hand lane of roadway 10 is below a pre-set maximum level. Thus,level monitor 38 requires only a single section 78. The output line frommonitor 38 to AND gate 40 is connected to the normally-closed relaycontact 114, and the potentiometer 84 is adjusted so that ground isapplied to this output line so long as the input on line 37 to levelmonitor 38 from volume computer 36 indicates that the volume of trafiicin the right-hand lane of roadway 10 is below the pre-set maximum level.

Level monitor 58 provides to gap detector 48 indications of the speed oftraffic passing through the detection zone of detector D2 in mergingarea 20. By way of example, these speeds might be divided into fourlevel ranges. Thus, three sections 78 are required for level monitor 58.The potentiometers 84 in each of the three sections are adjusted so thatthe output changes from output line L1 to output line L2 to output lineL3 to output line L4 as the speed indication from speed computer 56increases to show that the speed of vehicles passing through mergingarea 20 falls within the corresponding ranges.

Gap DetectorFIGURE 3 FIGURE 3 depicts a circuit suitable for use as gapdetector 48. Gap detector 48 receives an input on line 47 from vehicledetector D1 each time a vehicle passes detector D1 in the right-handlane of roadway 10. Gap detector 48 also receives inputs on lines 59from level monitor 58 which indicate the range of speed of vehiclespassing through the detection zone of detector D2 in merging area 20 asthe vehicles pass from access road 16 to merge into the traffic onroadway 10. The speed at which vehicles pass through the merging area20* is indicative of the ease with which the merging is performed. Thus,it indicates whether the gap in the traffic on roadway 10 is ofsufiicient length. This speed information is utilized by gap detector 48to adjust the length of the gap in the traflic on roadway 10 which mustbe detected before gap detector 48 generates output signals to initiateoperation of signal 24 to release a waiting or approaching vehicle, asby a given signal indication for example.

In the embodiment of gap detector 48 depicted in FIGURE 3, the input online 47 from vehicle detector D1 is applied to the first side of thecoil of relay 126, the second side of which is connected to a suitablesource of potential, such as +12 volts D.C. By way of example, the gapdetector of FIGURE 3 is suitable for use with a level detector 58 whichdivides into four speed level ranges, L1, L2, L3 and L4, the informationindicative of the speed of vehicles passing through merging area 20.Level monitor 58 accordingly is made up of three sections, eachidentical to section 78a of FIGURE 2. Should it be desired to divide thespeed information from speed computer 56' into more narrow ranges,additional level monitor sections such as section 78a are provided.

Each of the four input lines 59 bringing the inputs L1, L2, L3 and 1.4from level monitor 58 is connected to one side of a relay coil. Thus,inputs L1, L2, L3 and L4 are associated with relays 128, 130, 132 and134, respectively, within the gap detector 48. Each of the relay coils128, 130, 132 and 134 has its other side connected to the source ofpositive potential.

Each of the relays 126-134 has two sets of contacts. Relay 126 isillustrative of these. Relay 126 has a moving contact or armature 126awhich operates between normally-closed contact 126b and normally-opencontact 1260. Relay 126 has a second moving contact or armature 126dwhich operates between normally-closed contact 126e and normally-opencontact 126 Similarly, relays 128134 each have two armatures 128a-134aand 128d-134d, respectively, each of which operates betweennormally-closed contacts 128b-134b and 128e-134e, respectively, andnormally-open contacts 128c-134c and 128f-134f, respectively.

The armature 126a of relay 126 is connected to the first side ofresistor 136. Normally-closed contact 1261; is tied to the armature 128aof relay 128. Normally-open contact 1260 is tied to ground. Similarly,armature 126d of relay 126 is connected to the first side of resistor13.8. Normally-closed contact 1262 is tied to armature 128d of relay128. Normally-open contact 126 is tied to ground. Normally-closedcontact 12812 is tied to armature 130a of relay 130. Normally-opencontact 1280 is coupled through resistor 128g and rheostat 128k to thesource of positive potential. Normally-closed contact 128e is tied toarmature 130d. Normally-open contact 128]" is coupled through resistor128i and rheostat 128i to the source of positive potential.Normally-closed contacts 13% and 130a are tied to armatures 132a and132d,

respectively. Normally-open contact 130c is coupled to the source ofpositive potential through resistor 130g and rheostat 13011.Normally-open contact 130 is coupled to the source of positive voltagethrough resistor 130i and rheostat 1301'. In like manner,normally-closed contact 13212 is tied to armature 134a; normally-closedcontact 132:; is tied to armature 134d; normally-open contact 1320 iscoupled through resistor 132g and rheostat 13211 to the source ofpositive potential; and normally-open contact 132] is coupled throughresistor 132i and rheostat 132i to the source of positive potential.Normally-closed contacts 134k and 134e have no connections. Normallyopencontact 1340 is coupled through resistor 134g and rheostat 134h to thesource of positive potential, and normally-open contact 134 is coupledthrough resistor 134i and rheostat 134 to the source of positivepotential. If lever monitor 58 divided the speed information into morethan four ranges, then the gap detector of FIG- URE 3 would receiveadditional inputs on lines 59 from the level monitor. In such a case, itwould have additional relays such as the relays 128134, and the contactsof these additional relays would be added to the chain of relay contactsdepicted in FIGURE 3, joining the chain at contacts 134b and 1342.

The second side of resistor 136 is coupled through resistor 140 to thegrid of triode 142, which by way of example may be one-half of adual-triode tube. The junction of resistors 136 and 140 is connected tothe first plate of capacitor 144, which has its second plate tied toground. The cathode of triode 142 is coupled through resistor 146 toground. The plate of triode 142 is connected to the first side of thecoil of relay 148, which has its other side connected to a source ofpositive potential such as +120 volts D.C. Relay coil 148 is shunted bycapacitor 150.

Triode 152 which, for example, may be the other side of the samedual-triode tube, has its plate tied to the source of positivepotential, its cathode tied to the cathode of triode 142, and its gridconnected to the first side of resistor 154. The second side of resistor154 is connected to the plate of triode 142. The grid of triode 152 isalso coupled to ground through resistor 156, which is shunted bycapacitor 158. Armature 148a of relay 148 is tied to the source ofpositive potential, and normallyopen contact 148b is connected to outputline 49 on which is transmitted output signal G1, indicative of thedetection by vehicle detector D1 of a gap of duration sufficient for avehicle from entrance ramp 16 to merge with the traffic on roadway 10.

Resistor 138 couples armature 126d to an identical dual-triode circuit.Thus, the second side of resistor 138 is coupled through resistor 160 tothe grid of triode 162. The junction of resistors 138 and 160 isconnected to the first plate of capacitor 164, which has its secondplate tied to ground. The cathode of triode 162 is coupled throughresistor 166 to ground. The plate of triode 162 is tied to the firstside of the coil of relay 168, which has its second side connected to asource of positive potential, such as volts D.C. Relay coil is shuntedby capacitor 170.

Triode 172 has its plate tied to the source of positive potential, itscathode tied to the cathode of triode 162 and its grid coupled throughresistor 174 to the plate of triode 172. The grid of triode 172 iscoupled to ground through resistor 176, which is shunted by capacitor178. Armature 168:: of relay 168 is tied to a source of positivepotential, while normally-open contact 168b is connected to output line51, on which is transmitted output signal G2, indicative of thedetection by vehicle detector D1 of a long-duration gap of a lengthsuflicient for more than one vehicle from entrance ramp 16 to merge withthe traffic on roadway 10.

When an input is applied to relay 126 from vehicle detector D1, thefirst side of relay coil 126 is connected to ground, thereby energizingthe relay. Similarly, when level monitor 58 applies an input on one ofthe input lines 59 to one of the relays 128-134, the first side of thatrelay coil is connected to ground, thereby energizing the relay. Assumefor the moment that no inputs are applied to gap detector 48 from levelmonitor 58. Then relays 128-134 are all in their de-energized condition,and so no voltage is applied from armatures 126a and 126d to the twodual-triode circuits. In such a case, triode 152 conducts becausecurrent flowing from the positive voltage source through relay coil 148,resistor 154 and resistor 156 to ground biases the triode to conduction.Similarly, triode 172 conducts because of current flow through relaycoil 168 and resistors 174 and 176. However, because resistors 154, 156,174 and 176 are very large, the currents flowing through relay coils 148and 168 are not great enough to energize the relays.

When one of the relays 128-134 is energized, its normally-open contactsclose, providing charging paths for capacitors 144 and 164. As currentpasses through resistor 136, the voltage on capacitor 144 increases.This raises the voltage on the grid of triode 142 until that triodecommences to conduct. When that happens, the current flowing throughrelay 148 increases, and the relay is energized, thereby applyingvoltage from the positive voltage source through armature 148a andnormallyopen contact 148b to output line 49 to generate the outputsignal G1. Similarly, current passing through resistor 138 chargescapacitor 164 until triode 162 conducts, energizing relay 168 andapplying voltage from the positive voltage source through armature 168aand normallyopen contact 168b to output line 51 to generate outputsignal G2.

When the system is in operation, one and only one of the relays 128-134is energized at all times by one of the inputs L1-L4 on lines 59 to gapdetector 48 from level monitor 58. The relay which is energized pullsits armatures into contact with their normallyopen contacts. Thisprovides charging paths for ca aci- 'tors 144 and 164 from the positivevoltage source, through the rheostats ond resistors associated with therelay and through their normally-open contacts, through normally-closedcontacts 126b and 126e, through armatures 126a and 126d, and throughresistors 136 and 138 to the capacitors 144 and 164, and to ground.Thus, by way of example, if the speed of vehicles passing throughmerging area 20 is at a low level, the output voltage from speedcomputer 56 is low, and so level monitor 58 applies an input L1 to gapdetector 48. As

a consequence, relay 128 is energized, closing its normally-opencontacts.

The rates at which capacitors 144 and 164 charge are determined,respectively, by the settings of rheostats 128h and 128 When a vehiclepasses vehicle detector D1, relay 126 is energized, and so armaturcs126a and 126d are connected to ground through normallyopen contacts 1260and 126 respectively. This discharges capacitors 144 and 164. After thevehicle has passed detector D1, relay 126 is de-energized, and armatures126a and 126a. return to contact with normallyclosed contacts 126k and126e, respectively, and the charging of capacitors 144 and 164 commencesagain. Thus, the start of a gap between consecutive vehicles in theright-hand lane of roadway is marked by the commencement of the chargingof capacitors 144- and 164.

If the gap is of a long enough time duration, then before relay 126causes the discharge of capacitor 144, the charge on capacitor 144increases the voltage on the grid of triode 142 until the triode 142conducts. This energizes relay 148, causing an output G1 on line 49.Thus, if the gap between vehicles passing detector D1 is of a timelength sufficient for a vehicle from entrance ramp 16 to merge with thetraffic on roadway 10,'relay 148 is energized to provide output G1 online 49. If this gap continues until it is so long that additionalvehicles from ramp 16 could merge with the traffic on roadway 10, thenbefore relay 126 causes the discharge of capacitor 164, the charge oncapacitor 164 increases the voltage on the grid of triode 162 untiltriode 162 conducts. This energizes relay 168, and output G2 is providedon line 51. These outputs G1 and G2 on lines 49 and 51, respectively,are applied by g p detector 48 to gap projector 50 which determines whenthe gaps have reached the release point R.

Capacitor 144 charges as a function of time. Thus, a gap betweenvehicles of a minimum time length is required to generate output signalG1. As the speed of vehicles on roadway 10 increases, the distancelength of this minimum time length gap increases. Thus, gap detector 48automatically varies the distance length of the required gap inaccordance with the speed of the trafiic on roadway 10. This, of course,is modified by the effect of the inputs L1L4 to gap detector 48.

The speed at which vehicles travel through merging area indicates theease with which these vehicles are merging into the trafiic on roadway10. If traflic on roadway 18 is of low density and is moving atrelatively high speed, then vehicles entering this trafiic from entranceramp 16 pass through merging area 28 at a relatively high speed.However, when the density of the trafiic on roadway 10 is greater or thespeed of traffic o-n roadway 18 is lower, vehicles merging into thistraffic from entrance ramp 16 passes through merging area 20 at lowerspeeds.

If the speed at which the vehicles pass through merging area 20increases, then level monitor 58 removes input L1 applied to gapdetector 48 and replaces it with input L2. This tie-energizes relay 128,removes resistor 128g and rheostat 128k from the charging circuit ofcapacitor 144 and removes resistor 128i and rheostat 128 from thecharging circuit of capacitor 164. Relay 130 is energized and resistor130g and rheostat 130k and resistor 130i and rheostat 130j providecharging paths for capacitors 144 and 164, respectively. Rheostats 13%and 130i are adjusted to lower settings of resistance then are thecorresponding rheostats 128k and 128i. Consequently, the currentsflowing in the charging paths of capacitors 144 and 164 are greater whenlevel detector 58 applies input L2 to gap detector 48 than they are whendetector 58 applies input L1 to the gap detector. Therefore, relays 148and 168- are energized more quickly than before. Thus, the minimumacceptable gap between consecutive actuations of relay 126 by vehiclespassing vehicle detector D1 is shortened. Each time a gap of the minimumacceptable duration is detected, relay 148 is energized to generateoutput signal G1. Each time the detected gap is of a duration greatenough to admit more than one vehicle onto roadway 10, as determined bythe output level L1-L4 from level monitor 58 to gap detector 48, relay168 is energized to generate output signal G2.

Should the speed of vehicles passing through mergingarea 28 increasefurther, level monitor 58 applies signal L3 to gap detector 50, and sorelay 130 is deenergized and relay 132 is energized. This removesresistor 130g and rheostat 13% from the charging path of capacitor 144and removes resistor 130i and rheostat 130j from the charging path ofcapacitor 164. In place of these, resistors 132g and rheostat 132kprovide a charging path for capacitor 144, while resistor 132i andrheostat 132] provide a charging path for capacitor 164.

Again, rheostats 132/1 and 132 are adjusted to lower settings than theircorresponding rheostats 130k and 138 and so a higher current flows inthe charging paths of capacitors 144 and 164. Thus, shorter gaps areaccepted to give outputs G1 and G2. The speed level L4 signal from levelmonitor 58 to gap detector 50 energizes relay 134 and results in evenshorter gaps being accepted.

In the embodiment of gap detector 48 which is depicted in FIGURE 3, thespeed indications L1-L4 determine the gap durations required to generatesignals G1 and G2 by controlling the resistance in the charging paths ofcapacitors 144 and 164, respectively. Obviously, equivalent designscould be used. For example, the signals L1-L4 could be used to controlbias on triodes 142 and 162 to vary the amount by which the voltage oncapacitors 144 and 164 must be increased to cause the triodes 142 and162 to conduct. Alternatively, signals L1L4 could vary the supplyvoltage applied to one end of each of two fixed resistances which havetheir other ends connected respectively to relay contacts 126]) and1262. Other equivalent designs could be used.

Gap ProjectorFIGURE 4 Gap projector 50 receives from vehicle detector D1an indication on line 47 whenever a vehicle is not within the detectionrange of detector D1. This input is indicative of the passage of a gapbetween vehicles past detector D1. If that gap is of a length sufficientto allow a vehicle from entrance ramp 16 to merge into it on roadway 10,gap projector 50 receives signal G1 on line 49 from gap detector 48. Gapprojector 50 also is connected by lines 33 to level monitor 32 toreceive signals which are indicative of the speed at which trafiic istravelling in the right-hand lane of roadway 10. From this information,gap projector 58 predicts when the gap has reached release point R. Gapprojector 50 then generates on line 52 an output signal G1 having aduration equal to the duration of the detected gap. If gap detector 48indicates that the detected gap is of a great enough duration to allowmore than one vehicle to merge into the tratfic on roadway 10, then ittransmits signal G2 on line 51 to gap projector 58, and gap projector 50provides on line 54 an output G2 indicative of this long duration gap.

FIGURE 4 depicts circuitry suitable for use as gap projector 50. Inputline 47 to the gap projector of FIGURE 4 couples vehicle detector D1 tothe first plate of capacitor 188 which has its second place tied to theanode of diode 190. The cathode of diode 190 is connected to the inputof monostable multivibrator or one-shot 192. One-shot 192 has its outputconnected to the set input of flip-flop 194. The second plate ofcapacitor 188 is also connected to the cathode of diode 196, which hasits anode tied to the input of one-shot multivibrator 198. The output ofone-shot 198 is tied to the reset input of flip-flop 194. The firstplate of capacitor 188 is coupled through resistor 199 to a source ofpositive voltage such as +12 volts D.C.

Shift register 200 is made up of a number of interconnected stages200a200x, and the set output of flip-flop 194 is tied to the set inputof the first stage 200a. The reset input to each stage of shift register200 is connected to the output of one-shot 198. Shift register 202 ismade up of a large number of stages, 202a-202z, 202A202Z. The output ofeach stage of shift register 200 is coupled to the set input of thecorresponding stage of shift register 202 through AND gates 204. Thus,stage 20011 has its output connected to the first input of AND gate204a, the output of which is connected to the set input of stage 202a;the output of stage 200k is connected to the first input of AND gate204b which has its output tied to the set input of stage 202b; etc.Thus, for each stage in shift register 200 there is a corresponding ANDgate 204 and a corresponding stage in shift register 202. Shift register202, however, contains more stages than does shift register 200. Thesecond input of each AND gate 204 is connected to input line 49 whichbrings signal G1 from gap detector 48 to gap projector 50.

Controlled multivibrator 208 has ten input lines 33 bringing the teninputs L1L10 from the corresponding lines 33 of level monitor 32. Levelmonitor 32 is of the type described above with reference to FIGURE 2,and, in this representative example, contains nine additional sections,each identical to section 78a. As described above, one and only one ofthe outputs L1-L10 from level monitor 32 is present at any one time, andthat output which is present is represented by ground potential on itsoutput line. Controlled multivibrator 208 is a free-runningmultivibrator, the free-running rate of which is dependent upon whichone of the inputs L1-L10 from level monitor 32 is at ground potential.

The output of multivibrator 208 is tied to shift register 202 to providethe shift pulses for the shift register 202. The output of multivibrator208 is also connected to the signal input of INHIBITED-AND gate 210, theoutput of which is tied to shift register 200 to provide the shiftpulses to shift register 200. The inhibit input of INHIBIT- ED-AND gate210 is connected to input line 49 which brings the input signal G1 fromgap detector 48.

The output of shift register stage 202x is connected to the first inputof AND gate 2060 which has its other input tied to the input line 33which brings signal L10 from level monitor 32. The output of shiftregister stage 202A is connected to the first input of AND gate 6b whichhas its other input tied to the input line 33 which brings signal L9from level monitor 32. The output of shift register stage 202W isconnected to the first input of AND gate 206i which has its other inputtied to the input line 33 which brings signal L2 from level monitor 32.The output of shift register stage 202Z is connected to the first inputof AND gate 206 which has its other input tied to the input line 33which brings signal L1 from level monitor 32. Similarly, six otherstages of shift register 202 between stages 202A and 202W have theiroutputs connected to inputs of AND gates 206c-206h, respectively. Thesesix other gates 206c-206h have their second inputs connected to theinput lines 33 bringing signals L8-L3 respectively, from level monitor32. The output of each of the AND gates 206 is connected to OR gate 207.The output of OR gate 207 is tied to output line 52 on which outputsignal G1 is transmitted from gap projector 50 to INHIBITED- AND gate 53(FIGURE 1) to signify that the detected gap has reached release point R.

Input line 51 brings the input signal G2 from gap detector 48 to gapprojector 50 where it is applied to the set input of flip-flop 216. Theset output of flip-flop 216 is connected to the first input of AND gate218, which has its second input connected to the output line 52. Theoutput of AND gate 218 is connected to the output line 54 on which theoutput signal G2 is transmitted to signify that the gap which hasreached release point R has a duration long enough to let more than onecar from entrance ramp 16 merge with the traffic on roadway 10.

Output line 54 is also connected to the input of inverting amplifier222, which has its output connected to the first plate of capacitor 224.The second plate of capacitor 224 is connected to the anode of diode 226and to the cathode of diode 228. The cathode of diode 226 is tied to thereset input of flip-flop 216. The anode of diode 228 is coupled toground through resistor 230. The anode of diode 226 is coupled to groundthrough resistor 232.

The speed at which vehicles are traveling on roadway 10 as they passvehicle detector D1 determines which one of the inputs L1-L10 is appliedvia the associated line 33 from level monitor 32 to controlledmultivibrator 208. The pulse rate of the output from controlledmultivibrator 208 is thus dependent upon the level of the output voltagefrom speed computer 30 as monitored by level monitor 32. If the speed ofvehicles passing vehicle detector D1 is low, then the output voltagefrom speed computer 30 is low, and so level monitor 32 applies signal L1to controlled multivibrator 208. This causes multivibrator 208 to pulseat a low repetition rate. If the speed of vehicles on roadway 10 passingvehicle detector D1 increases, then the output voltage from speedcomputer 30 to level monitor 32 increases, and level monitor 32 removessignal L1 and replaces it with a higher level signal such as signal L2.As a consequence, the repetition rate of controlled multivibrator 208increases.

When a vehicle is within the detection zone of vehicle detector D1,ground is applied to input line 47 of gap projector 50. Thus, each gapbetween vehicles is indicated on input line 47 by the removal of ground.The resulting positive-going pulse passes through capacitor 188 anddiode 190 to trigger one-shot multivibrator 192. The output pulse ofone-shot 192 sets flip-flop 194. The set output of flip-flop 194 setsshift register stage 200a to signify the presence of a gap at vehicledetector D1. The first pulse from multivibrator 208 after stage 200a isset shifts the set condition from stage 200a to stage 20%. Sinceflip-flop 194 is still in its set condition, stage 200a remains set.With each succeeding pulse from multivibrator 208, the set conditionshifts to the next succeeding stage of shift register 200, and the setoutput of flipflop 194 re-introduces a set condition at the first stage200a.

If the gap is of a duration too short to permit a vehicle to merge intothe traffic on roadway 10 from entrance ramp 16, then input signal G1 isnever applied from gap detector 48 to gap projector 50 on line 49. Whena vehicle passes vehicle detector D1 to end the gap, ground is appliedon line 47, and the resulting negative-going pulse passes throughcapacitor 188 and diode 196 to trigger one-shot multivibrator 198. Theoutput of one-shot 198 resets flip-flop 194 and resets each stage ofshift register 200 so that the entire shift register is resetsimultaneously. When that vehicle has passed from the detection zone ofvehicle detector D1, another gap occurs and ground is removed from inputline 47. As a consequence, one-shot 192 sets flip-flop 194 which appliesa signal to the set input of shift register stage 200a. Again with eachpulse from multivibrator 208, the set condition is transferred to thenext succeeding shift register stage and flip-flop 194 retains stage200a in its set condition. If this gap is of a duration sufficient topermit a vehicle from entrance ramp 16 to merge into the traffic onroadway 10, then gap detector 48 applies signal G1 to gap projector 50by means of input line 49. Signal G1 is applied as an enabling input toeach AND gate 204.

The AND gates 204 read the information from shift register 200 and writeit into the corresponding stages of shift register 202. Thus, if the setcondition of shift register 200 has progressed, for example, to stage200g by the time that input signal G1 is applied on lnput l1ne 49, theneach of the shift register stages 200a-200g is in its set condition atthat time, while the succeeding stages 200k, etc., are not. AND gates204a-204g pass this set condition to the set inputs of shift registerstages 202a-202g. AND gates 204k, etc. are also enabled by signal G1,but shift register stages 2001:, etc. are not set, and so no set inputis applied to shift register stages 202k, etc. Each succeeding pulsefrom multivibrator 208 transfers the gap indication to the succeedingstages of shift register 202 until the set condition reaches the laststage 202Z.

Input line 49 is connected to the inhibit input of INHIBITED-AND gate210 so that pulses from multivibrator 208 do not shift the informationwithin shift register 200 after signal G1 is received. Consequently,after input signal G1 is received, the output of stage 200a remains set,and so it continues to apply a set input to stage 202a at each pulsefrom multivibrator 208. When the next vehicle passes vehicle detectorD1, ground is again applied on input line 47, and one-shot 198 resetsflip-flop 194 and each of the stages of shift register 200. At that timethe signal G1 is also removed from input line 49. As a consequence, theset input is no longer applied to shift register 202a and so, with eachsucceeding pulse from multivibrator 208, the reset condition of theshift register is transferred to the succeeding stages, marking the endof the gap.

The speed of vehicles passing detector D1 determines which input Ll-L isapplied via lines 33 from level monitor 32 to gap projector 50.Whichever input L1L10 is present enables its corresponding AND gate 206.When the gap indication passing through shift register 202 reaches thestage which has its output tied to the enabled AND gate 206, a signalpasses from that AND gate 206, through OR gate 207 to line 52 toindicate that the gap has reached release point R on roadway 10. Thisoutput signal G1 on line 52 continues so long as the shift registerstage within shift register 202 which is providing it continues in itsset condition. Thus, it continues until the reset condition has beenshifted to that stage of shift register 202, following the terminationof the gap. Accordingly, the gap indication output G1 on line 52 existsfor the same length of time that the gap itself exists, as indicated bythe potential on input line 47 from detector D1.

If the gap is of a duration sufiicient to permit more than one vehiclefrom entrance ramp 16 to merge into it on roadway 10, then signal G2 isgenerated by gap detector 48 and is applied by line 51 to gap detector50. This sets flip-flop 216, and the set output of flip-flop 216 enablesAND gate 218. Consequently, when the output signal G1 is provided onoutput line 52, it is applied to AND gate 218, causing AND gate 218 togenerate output signal G2 on output line 54. The resulting negativepulse from inverting amplifier 222 is passed to ground through diode 228and resistor 230. After output signal G1 terminates, indicating that theend of the gap has reached release point R, the input is removed fromAND gate 218, and so output signal G2 ends. This results in apositive-going pulse passing from inverting amplifier 222 throughcapacitor 224 and'diode 226 to reset flip-flop 216.

With gap detector 50 operating in this manner, the number of stages inshift register 202 through which the gap indication passes, commencingat stage 202a, is proportional to the distance on roadway 10 fromvehicle detector D1 to release point R.

When traffic on roadway 10 is travelling at a high speed and isclosely-spaced, it is likely that traffic through merging area 20 istravelling at a low speed. In such a situation, level monitor 58 appliessignal L1 to gap detector 48. This requires a long-duration gap betweenvehicles passing vehicle detector D1 before gap detector 48 generatessignal G1. The high speed of vehicles passing vehicle detector D1results in level monitor 32 applying signal L10 to gap projector 50 tocause a high output rate from controlled multivibrator 208.Consequently, the gap indication is shifted through shift register 200at a rapid rate. Shift register 200 must contain enough stages to permitstorage of the entire gap at this highest repetition rate, up untilsignal G1 is received.

Shift register 202 must contain enough stages to permit delay of the gapsignal for the time required for the gap to reach release point R. Asthe speed of vehicles passing vehicle detector D1 increases, the speedwith which the gap reaches release point R increases. However, therepetition rate of controlled multivibrator 208 also increases, and sothe gap signal is passed through shift register 202 more rapidly. Thus,the number of stages required in shift register 202 is dependent uponthe distance which vehicle detector D1 is located from merging area 20.While FIGURE 4 indicates that shift register 200 includes stages200a-200x and that shift register 202 includes stages 200a200z, this isonly a representation intended to indicate that a large number of stagesexist, and it is not intended in any way to indicate the exact number ofstages in either shift register.

The gap indication within shift register 202 is not reset when a vehicleindication on input line 47 triggers oneshot multivibrator 198. Onlyflip-flop 194 and shift register 200 are reset. At the next pulse fromcontrolled multivibrator 208, shift register stage 202a is reset, andwith each succeeding pulse from multivibrator 208 this reset conditionpasses to a succeeding stage within shift register 202, marking passageof the end of the gap along roadway 10. When the vehicle indicationends, the signal on line 47 triggers one-shot 192, which sets flip-flop194, to start the next gap indication through shift register 200. Ifthis gap is found to be long enough to permit merger of a vehicle intoit, signal G1 is again applied to AND gates 204 to transfer the gapindication from shift register 200 to shift register 202. Thus, passageof two or more gaps along roadway 10 can be predicted simultaneously bygap projector 50.

Controlled multivibrator 208 might be any conventional design offree-running multivibrator having a repetition rate determined by whichone of the .inputs L1 through L10 is applied to it. By way of example,the controlled multivibrator 208 could be a standard free-runningmultivibrator with each input line L1 through L10 connected to acorresponding relay. Then application of ground potential by levelmonitor 32 to one of the input lines L1L10 energizes that correspondingrelay to place a corresponding resistor in the timing circuit of thefree-running multivibrator. Alternatively, solid state switching couldbe utilized in place of relays.

Signal Sequence Timer and ControllerFIGURE 5 Signal sequence timer andcontroller 44 receives input signals on lines 42, 54, 64, 66 and 70 fromthe vehicle detectors and control circuitry and, in response to thesesignals, controller 44 applies voltages via line 46 to control signal 24to cause signal 24 to indicate to vehicles waiting on entrance ramp 16when they should proceed toward merging area 20. For example, controller44 can be an electro-mechanical device, made up of appropriate timingand gating circuitry and a stepping switch. Alternatively, controller 44can comprise electronic switching circuits which provide the requiredsequence of signals.

The particular design of controller 44 is dependent in part upon thetype of trafiic control signal 24 utilized. FIGURE 5 depicts anelectronic controller which is suitable for use as signal sequence timerand controller 44 when signal 24 is of the well-known red, yellow, greentraffic light type and which is made up of standard components.

Input line 42 from the output of AND gate 40 (FIG- URE 1) to signalsequence timer and controller 44 is connected to the first input of ORgate 238 which has its other input tied to line 54 on which signal G2 istransmitted by gap projector 50. Input line 42 is also tied to the firstinput of OR gate 240. Line 66 from the output of INHIBITED-AND gate 53(FIGURE 1) is connected to the second input of OR gate 240. The outputof OR gate 240 is connected to the first input of AND gate 246 which hasits output tied to the first input of OR gate 248. Line 70 from theoutput of INHIBITED-AND gate 62 (FIGURE 1) is connected to the firstinput of AND gate 250 within controller 44. The output of AND gate 250is tied to the input of time delay circuit 252 which has its outputconnected to the second input of OR gate 248. The output of OR gate 248is connected to the reset input of flip-flop 254 and to the set input offlip-flop 256.

The output of OR gate 238 is tied to the cathode of diode 257 which hasits anode connected to the input of one-shot multivibrator 258. Theoutput of one-shot 258 is connected to the first input of OR gate 259.Line 64 from detector D3 is connected to the input of invertingamplifier 260 which has its output tied to the second input of OR gate259. The output of OR gate 259 is connected to the first signal input ofINHIBITED-AND gate 261 which has its inhibit input tied to the output ofOR gate 238. The output of INHIBITED-AND gate 261, is connected to thereset input of flip-flop 256 and to the set input of flip-flop 262.

The set output of flip-flop 254 is connected to control signal 24 byline 264 to provide the red or stop indication for the control signal24. The set output of flip-flop 256 is connected to control signal 24 byline 266 to provide the green or go indication for the control signal24. The set output of flip-flop 262 is connected to control signal 24 byline 268 to provide the yellow or clearance indication for the controlsignal 24. These output lines 264, 266 and 268 from flip-flops 254, 256and 262, respectively, constitute output line 46, shown in FIGURE 1connecting controller 44 to control signal 24.

Line 264 from the set output of flip-flop 254 is connected to the inputof timer circuit 270, which has its output tied to the second input ofAND gate 246 and to the second input of AND gate 250. Line 266 from theset output of flip-flop 256 is connected to the second signal input ofINHIBITED-AND gate 261 and to the first input of AND gate 274. AND gate274 has its output connected to the input of time delay circuit 276, theoutput of which is connected to the third input of OR gate 259. Line 268from the set output of flip-flop 262 is connected to the input of timercircuit 278 and to the input of timer circuit 280. The output of timercircuit 278 is applied to the first input of AND gate 282. The output oftimer circuit 280 is connected to the first signal input ofINHIBITED-AND gate 284. The outputs of gates 282 and 284 are connectedto the two inputs of OR gate 286. The output of OR gate 286 is tied tothe input of one-shot 287 which has its output connected to the setinput of flip-flop 254 and to the reset input of flip-flop 262.

Input lines 66 and 70 are connected to the two inputs of OR gate 288,which has its output tied to the set input of flip-flop 290. The setoutput of flip-flop 290' is connected to the second signal input ofINHIBITED-AND gate 284 and to the signal input of INHIBITED-AND gate291. The reset input of flip-flop 290 is tied to the output of one-shot287. The output of OR gate 238 is connected to the set input offlip-flop 292 which has its reset input tied to the output of one-shot287 and its set output connected to the second input of AND gate 282 andto the inhibit of INHIBITED-AND gate 284. Input line 70 is connected tothe inhibit input of INHIBITED- AND gate 291, which has its output tiedto the second input of AND gate 274.

OPERATION OF THIS PREFERRED EMBODIMENT The control system of the presentinvention is capable of adapting its operation to meet the varyingtraffic conditions which occur on limited-access roadway 10, entranceramp 16, and frontage road 18. Its primary manner of operation is usedwhen traffic on roadway is heavy and the number of vehicles on entranceramp 16 waiting to enter roadway 10 from frontage road 18 is high, forexample, during the periods in the morning and evening when the commutertraflic is heavy. Under such conditions, volume computer 36 provides anoutput to level monitor 38 which indicates that the volume of trafficpassing vehicle detector D1 is high. As a consequence, level monitor 38does not provide an output to AND gate 40, and so no input on line 42 isprovided to signal sequence timer and controller 44.

Gaps between consecutive vehicles in the right-hand lane of roadway 10are detected by gap detector 48. Speed computer 56 provides to levelmonitor 58 a voltage indicative of the speed with which vehicles havebeen passing through merging area 20. As determined by this voltagelevel, level monitor 58 provides one of the signals L1L4 to gap detector48, thereby grounding the associated relay 128-134 within the gapdetector. During the gaps in the traffic on roadway 10, the timingcircuit associated with relay 148 within gap detector 48 commences totime. If another vehicle comes within the detection zone of vehicledetector D1 before relay 148 is energized, then energization of relay126 within gap detector 48 terminates the timing operation bydischarging capacitor 144 through resistor 136, armature 126a, andnormally-open contact 1260 to ground. When a gap passes detector D1 witha duration great enough to permit a vehicle from entrance ramp 16 tomerge onto roadway 10, the voltage on capacitor 144 increases untiltriode 142 conducts. As a result, relay 148 is energized, generatingsignal G1 which is transmitted to gap projector 50 on line 49. The timerequired for the voltage on capacitor 144 to cause initiation of signalG1 is, of course, dependent upon which input L1-L4 is applied to gapdetector 48'from level monitor 58.

The signal from vehicle detector D1 is also applied to gap projector 50via line 47. As a result, flip-flop 194 within gap projector 50 setsshift register stage 200a. Pulses from controlled multivibrator 208within gap projector 50 shift this set condition through the consecutivestages of shift register 200. The speed at which controlledmultivibrator 208 causes this set condition, indicative of the detectedgap, to be shifted through shift register 200 is dependent upon thespeed at which vehicles have been passing vehicle detector D1. Thisvehicle speed is converted to a voltage by speed computer 30, and thevoltage is applied to the input of level monitor 32 which provides oneof the indications L1L10 to controlled multivibrator 208, dependent uponthe level of the voltage from speed computer 30. The repetition rate ofcontrolled multivibrator 208 is dependent upon which one of theindications L1-L10 is applied to the controlled multivibrator from levelmonitor 32. Accordingly, the rate at which the gap indication is shiftedthrough shift register 200 is dependent upon the speed of traffic onroadway 10.

When gap detector 48 applies signal G1 to gap projector 50 on line 49,the AND gates 204 transfer the gap indication from shift register 200 toshift register 202. The shift pulses from multivibrator 208 are blockedby INHIBITED-AND gate 210, and so there is no signal ap plied to theshift input of shift register 200. Shift register stage 200a remains inits set condition as long as the gap exists, and this set output fromstage 200a is passed through AND gate 204a to the set input of shiftregister stage 202a.

The entire gap indication from shift register 200 is transferred toshift register 202. Pulses from multivibrator 208 cause this gapindication to be shifted through the consecutive stages of shiftregister 202a. With each shift of information through the stages ofshift register 202, the set signal from stage 200a through AND gate 204acauses stage 202a to remain in its set condition, and so the gapindication is continuously fed into shift register 202, so long as thegap continues. Once the gap ends, signal G1 is terminated, removing theenabling inputs from AND gates 204, and one-shot 198 resets flip-flop194 and resets each stage of shift register 200. The next pulse frommultivibrator 208 causes shift register stage 202a to resume its resetcondition, and with succeeding pulses from multivibrator 208 thesucceeding stages of shift register 202 are reset, indicating thepassage of the end of the gap through the shift register 202.

The gap indication continues to progress through the consecutive stagesof shift register 202 with each pulse from controlled multivibrator 208.That input signal L1 L which is present on line 33 enables itsassociated AND gate 206. When the set condition indicative of theleading edge of the gap reaches the shift register stage which isconnected to the enabled AND gate 206, output signal G1 is initiated online 52. This signal is applied to INHIBITED-AND gate 53. If no vehicleis stopped within the detection zone of detector D2 in merging area andif a vehicle is waiting within the detection zone of detector D4 infront of control signal 24, then INHIB- ITED-AND gate 53 applies signalG1 on line 66 to signal sequence timer and controller 44.

The signal G1 on line 66 passes through OR gate 240 to the first inputof AND gate 246. If control signal 24 is indicating red or stop, thenflip-flop 254 is set, and this set output is applied to the input oftimer 270 which times a minimum duration for the red indication. Oncethe minimum red duration has passed, timer 270 applies an input to ANDgate 246. Coincidence of the two inputs to AND gate 246 results in asignal passing from AND gate 246 through OR gate 248 to reset flipflop254 and to set flip-flop 256. As a consequence, the red or stopindication on line 264 from the set output of flip-flop 254 isterminated, and it is replaced with a green or go indication on line 266from the set output of flip-flop 256. Thus, control s gnal 24 indicatesgreen or go to release the vehicle waiting on entrance ramp 16 in frontof the control signal 24.

The vehicle on entrance ramp 16 then proceeds toward merging area 20 tomerge onto roadway 10. When that vehicle passes over detector D3 on itsway toward merging area 20, ground is applied on line 64 to the input ofinverting amplifier 260 within controller 44. Inverting amplifier 260then applies a signal through OR gate 259 to the first signal input ofINHIBITED-AND gate 261. The second signal input of INHIBITED-AND gate261 is energized by the green indication on line 266 from the set outputof flip-flop 256. There is no signal applied to the inhibit input ofINHIBITED-AND gate 261, and so the INHIBITED-AND gate applies a signalto the reset input of flip-flop 256 and to the set input of flip-flop262. As a consequence, the green or go indication on line 266 from theset output of flip-flop 256 is terminated, and it is replaced by ayellow or clearance indication on output line 268 from the set output offlip-flop 262.

The input G1 on line 66, which initially caused the change of thecontrol signal from red to green, also passes through OR gate 288 to setflip-flop 290. The set output of flip-flop 290 is applied to the firstsignal input of IN- HIBITED-AND gate 284. The set output from flip-flop262, which provides the yellow indication for control signal 24, isapplied to the input of timer 280 which times a short duration yellow orclearance interval. When timer 280 has timed out, a signal is appliedfrom it to the second input of INHIBITED-AND gate 284. Since flip-flop292 is reset, there is no signal applied to the inhibit input ofINHIBITED-AND gate 284. Therefore, a signal from INHIBITED-AND gate 284,passes through OR gate 286 to trigger one-shot 287 which resetsflip-flops 262 and 290 and sets flip-flop 264. Consequently, the yellowor clearance indication from the set output of flip-flop 262 isterminated, and the red or stop indication is again provided by the setoutput of flip-flop 254. Thus, the control signal 24 is cycled from redto green to yellow and back to red to signal to the waiting vehicle thatit should proceed on entrance ramp 16 through merging area 20 to mergewith the trafiic on roadway 10.

Vehicle detector D1 applies a signal to gap detector 48 and to gapprojector 50 when the gap in the traflic on roadway 10 reaches thevehicle detector D1. During the time that gap detector 48 is determiningwhether this gap is great enough to permit merger into it of a vehiclefrom entrance ramp 16, the gap is moving along roadway 10 toward mergingarea 20 at the same speed that traflic is moving along roadway 10.'Thisprogress of the ga along roadway 10 is marked by the progression of thegap indication through shift register 200 at a rate dependent upon thespeed of traffic on roadway 10, as indicated by speed computer 30 andlevel monitor 32. Thus, by the time that gap detector 48 generatessignal G1 on line 49, the front edge of the gap has proceeded downroadway 10 to a decision point indicated in FIGURE 1 as point P. At thattime, the ga indication is transferred from shift register 200 to shiftregister 202 within gap projector 50. Shift register 200 must containenough stages to store the entire gap indication for a gap of themaximum length required to generate signal G1. This gap length isdetermined by which indication L1-L4 is applied from level monitor 58 togap detector 48, and the rate at which it passes through shift register200 is determined by which indication L1-L10 is applied from levelmonitor 32 to controlled multivibrator 208 via lines 33.

As the gap continues down roadway 10 toward merging area 20, the gapindication progresses through shift register 202, and when theindication reaches the last shift register stage 202Z, to transmitsignal G1 on line 52, the front edge of the gap has reached a releasepoint on roadway 10, indicated in FIGURE 1 at point R. Signal sequencetimer and conroller 44 then generates the green or go indication onoutput line 266 to cause control signal 24 to release the waitingvehicle on entrance ramp 16. This vehicle accelerates on entrance ramp16, and it must reach merging area 20 when the gap is at the mergingarea 20. Thus, the release point R must be located so that the gap,moving at then-same rate as traflic on road way 10, and the vehicle onentrance ramp 16, accelerating from rest in front of control signal 24,both require approximately the same amount of time to travel to mergingarea 20.

The rate at which the gap proceeds down roadway 10 from release point Rto merging area 20 varies with the speed of trafiic on roadway 10.However, since the vehicle on entrance ramp 16 must accelerate from infront of control signal 24 to merging area 20 while the gap is movingfrom release point R to merging area 20, the length of time which passesbetween arrival of the gap at lease point R and arrival of the gap atmerging area 20 is substantially constant. Thus, the distance fromrelease point R to merging area 20 increases as the speed of trafiic onroadway 10-increases. Accordingly, inputs L1-L10 are applied as enablinginputs to the AND gates 206 in an inverse order. Signal L10,representing a high speed of traffic on roadway 10, enables AND gate206a which-provides signal G1 on output line 52 when the gap is asubstantial distance away from merging area 20. Each subsequent inputsignal L9L1 enables an associated AND gate 206 which provides signal G1at a later time, when the gap is closer to merging area 20. Thus, thetime available for a vehicle to accelerate on ramp 16 to meet the gap atmerging area 20 is maintained substantially constant.

While FIGURE 4 depicts AND gates 206a, 206b, 206i and 206i associatedwith particular stages of shift register 202, this is only arepresentative example. The particular stage with which each AND gate206 is assoicated depends upon the characteristics of the entrance rampat which the control system is utilized, and to provide increasedflexibility, the AND gates 206 can be selectively connected to shiftregister 202 by means of patch cords or switching. Selector switches,for example, could be 23 calibrated in terms of percentage of totaldistance from D1 to nearly the merging area.

An alternative arrangement for providing a fixed acceleration timebetween the generation of signal G1 on line 52 and arrival of the gap atmerging area 20 is to select the values of resistance placed in thetiming circuitry of controlled multivibrator 208 by the signals L1-L10so that the multivibrator repetition rate increases with increased speedon roadway in a manner which shifts the gap indication through shiftregisters 200 and 202 at a rate to bring the indication to the output ofstage 202Z at the appropriate release time. In such a case, output line52 is tied only to the output of shift register stage 202Z.

Since shift register 200 and 202 simply delay signal Gl-Gl' and signalG2-G2' in accordance with the speed of traffic on roadway 10, a suitablyadjusted time delay under the control of the traffic speed may be usedto provide outputs G1 and G2 at suitable times after generation ofsignals G1 and G2. To accommodate a plurality of sequential gapsexisting between detector D1 and release point R simultaneously, anumber of time delay circuits can be arranged to be used in a mannersimilar to a telephone exchange line finder. Similarly, the gap could beplaced into a computer-type memory device as a given real time clockvalue Which could be arranged for look up at the particular timecorresponding to the location of release point R as determined by thetraffic speed on roadway 10.

Vehicle detector D1 must be far enough upstream from merging area toinsure that the decision point P is upstream of the release point R forthe maximum trafiic speed on roadway 10 and the minimum speed ofvehicles through merging area 20. Thus, the distance which detector D1is located upstream of merging area 20 is dependent upon the maximumspeed at which trafiic moves on roadway 10, the minimum speed ofvehicles through merging area 20, the distance from control signal 24 tomerging area 20, and the acceleration rate of vehicles on entrance ramp16. This distance, thus, must be determined for each entrance ramp.

If one acceptable gap is ended by passage of a single vehicle pastvehicle detector D1 and a second acceptable gap then follows, controller44 might be providing a yellow or clearance indication to control signal24 via line 268 at the time the second G1 signal is applied on line 66.Consequently, there will be no input applied to AND gate 246 from timer270 at the time the G1 signal passes through OR gate 240 to the otherinput of AND gate 246. Timer 270 insures that the red indication on line264 exists for at least a minimum time so that vehicles on ramp 16 willnot be too closely spaced as they pass through merging area 20. By thetime that timer 270 applies an enabling input to AND gate 246, thesecond gap may have terminated. In such a case, the G1 signal on line 66through OR gate 240 to AND gate 246 will have ended. Consequently, thegreen indication is not initiated. This prevents the system fromsignalling to a car to proceed to merging area 20 too late to merge intothe gap.

Timer 270 is reset when the input signal to it from line 264 ends. Thereset circuitry of timer 270 includes apparatus such as a one-shotmultivibrator which delays the resetting of the timer to insure that thetimer output has reset flip-flop 254 before the timer 270 output ends.

If the driver of the vehicle waiting in front of indicator 24 onentrance ramp 16 hesitates instead of proceeding on the entrance ramp 16once control signal 24 releases him to advance toward merging area 20,it is possible that the gap in the traffic on roadway 10 into which heis to merge will have passed merging area 20 before his vehicle gets tothe merging area. Therefore, the green indication on line 266 from theset output of flip-flop 256 is applied to one input of AND gate 274.Since 24 there is no signal on input line 70, INHIBITED-AND gate 291 isnot blocked. The set output from flip-flop 290 is applied throughINHlBITED-AND gate 291 to the other input of AND gate 274. Coincidenceof these two inputs to AND gate 274 causes an output from the AND gate274 which is applied to timer 276. Once this timer has timed out, asignal is applied fom it through OR gate 259 to the first signal inputof INHIBITED AND gate 261. This input to INHIBITED-AND gate 261 causesthe same sequence as would a signal from vehicle detector D3. Thus, thecontrol signal is cycled from its green indication through yellow andback to red, and the hesiiating driver must wait for another gap toappear in the traffic on roadway 10.

If the gap in the traffic on roadway 10 is of a duration long enough topermit more than one car to merge into the traffic on roadway 10, thenwithin gap detector 48 the voltage on capacitor 164 increases untiltriode 162 conducts, energizing relay 168 and causing output signal G2on line 51. Signal G2 is transmitted to gap projector 50 in which itsets flip-flop 216. The set output of flip-flop 216 enables AND gate218. When the output signal G1 is generated on line 52 and is applied tothe second input of AND gate 218, output signal G2 is generated on line'54.

Signal G2 is supplied by line 54 to signal sequence timer and controller44 in which it passes through OR gate 238 to inhibit INHIBITED-AND gate261. Thus, if the green indication is being transmitted on output line266 from flip-flop 256 when signal G2 reaches controller 44, controller44 is inhibited from proceeding to the yellow and the red indications.

If at the time signal G2 reaches controller 44, the controller is notproviding the green indication on its output line 266, then thecontroller continues its sequence until timer 270 has timed the mini-mumred duration, and an enabling input is applied by timer 270 to AND gate246. The G1 signal on line 52 is present at all times that the G2 signalis present on line 54. Consequently, if a vehicle is Within thedetection zone of vehicle detector D4, and no vehicle is stopped withinthe detection zone of vehicle detector D2. INHIBITED-AND gate 53 appliesa G1 signal on line 66 to cause controller 44 to step from the redindicator on line 264 to the green indication on line 266, as describedabove. However, the G2 signal on line 54 passes through OR gate 238 tothe inhibit input of INHIBITED-AND gate 261. Therefore, the greenindication continues for as long as the signal G2 is present, a dvehicles on entrance ramp 16 are continuously released toward mergingarea 20.

When the gap ends and the end of the gap has been transmitted throughall of the stages of shift register 202 within gap projector 50, signalG2 ends, resetting flipflop 216 within gap projector 50. The negativepulse marking the termination of signal G2 passes through diode 257 totrigger one-shot 258. The output from oneshot 258 passes through OR gate259 to INHIBITED- AND gate 261. Since the G2 signal is no longer presentand the green indication on line 266 is present, IN- HIBITED-AND gate261 provides a signal which resets flip-flop 256 and sets flip-flop 262.This terminates the green indication on output line 266 and initiatesthe yellow indication on output line 268.

After passing through OR gate 238, the G2 signal sets flip-flop 292which enables AND gate 282 and inhibits INHIBITED-AND gate 284. Thus,the set output of flip-flop 262 passes through timer 278 which times along duration yellow or clearance interval before applying a signal tothe other input of AND gate 282. Coincidence of the two inputs to ANDgate 282 results in a signal from gate 282 passing through OR gate 286to trigger one-shot 287. The one-shot output resets flip-flops 262 and292 and sets flip-flop 254. Thus, a long duration gap in the traffic onroadway 10 results in the signal G2 which permits the continuous releaseof vehicles until the end of the gap, at which time a long yellow orclearance signal is provided before the control 24 returns to its redindication.

When the traffic on limited-access roadway is light, the volume is lowand the traffic travels at a relatively high speed. Therefore, levelmonitor 38 provides a signal to AND gate 40 to indicate that the volumeof trafiic passing vehicle detector D1 is low, and level monitor 34provides a signal to AND gate 40 to indicate that the speed of vehiclespassing vehicle detector D1 is high. Coincidence of these two signals atAND gate 40 causes the AND gate to provide a voltage on line 42 tosignal sequence timer and controller 44 in which the voltage passesthrough OR gate 240. This signal through OR gate 240 causes controller44 to cycle to its green output indication on line 266, just as did thesignal G1 which passed through OR gate 240 on line 66. Similarly, thissignal on line 42 passes through OR gate 238 to the inhibit input ofINHIBITED-AND gate 261 so that the green or go indication fromcontroller 44 continuously permits release of vehicles.

When the traffic volume increases or the traffic speed decreases so thatthere is no longer a coincidence of inputs at AND gate 40, the signal online 42 is terminated. The resulting negative-going pulse passes throughdiode 257 to trigger out-shot multivibrator 258. The output fromone-shot 258 passes through OR gate 259 to IN- HIBITED-AND gate 261.Since there is no longer a signal on line 42, there is no input at theinhibit input of IN- HIBITED-AND gate 261. The green indication fromline 266 is applied to lNHIBITED-AND gate 261, and so the gate 261provides a signal which resets flip-flop 256 and which sets flip-flop262. The signal on line 42 passes through OR gate 238 to set flip-flop292. The output of flip-flop 292 enables AND gate 282 and blocks IN-HIBITED-AND gate 284. Consequently, timer 278 times a long durationyellow or clearance interval before an output from AND gate 282 passesthrough OR gate 286 to trigger one-shot 287 which resets flip-flops 262and 292 and sets flip-flop 254, returning the controller to its redindication.

If the trafiic flow on limited-access roadway 10 is extremely heavy,then it is possible that long periods of time will pass without a gapbeing detected with a duration sufiicient to permit a vehicle fromentrance ramp 16 to merge into it. If a large number of vehicles fromfrontage road 18 want to enter limited-access roadway 10 by means ofentrance ramp 16, then a line of waiting vehicles will form in front ofcontrol signal 24. If this line becomes long enough, it will reachturning area 22 adjacent the traveled portion of frontage road 18, and avehicle will stop within the detection zone of detector D5. If vehiclescontinue to line up waiting to enter entrance ramp 16, they will form aline on frontage road 18, and as a result the traflic on the frontageroad will be impeded.

To prevent interference with the trafiic on frontage road 18, an inputis applied to timer 68 when a vehicle is within the detection zone ofvehicle detector D5. Should that input continue for a long period oftime, as is the case when a vehicle stops within the detection zone ofdetector D5, timer 68 applies a signal to INHIBITED- AND gtae 62. Ifthere is also a vehicle within the detection zone of detector D4,immediately in front of control signal 24, then detector D4 applies aninput to IN- HIBITED-AND gate 62. If no vehicle is waiting within thedetection zone of detector D2 in merging area 20, then no inhibit inputis applied to INHIBITED-AND gate 62, and the gate provides a signal online 70 to controller 44.

This signal on line 70 is applied to AND gate 250 within controller 44.Since the controller is indictaing red, the timer 270 is applying anoutput to the second input of AND gate 250. Gate 250 therefore applies asignal to the input of time delay circuit 252. When this circuit hastimed out, a signal from it passes through OR gate 248 to resetflip-flop 254 and to set flip-flop 256.

As a consequence, the red indication on output line 264 from controller44 terminates, and the green indication on output line 266 is initiated.The vehicle at the front of the line waiting before control signal 24 isthen released to proceed on entrance ramp 16 toward merging area 20.When this vehicle passes over detector D3, ground is applied on line 64to the input of inverting amplifier 260 and so a signal passes throughOR gate 259 and INHIBITED-AND gate 261 to reset flip-flop 256 and to setflip-flop 262. This terminates the green indication and initiates theyellow indilation.

The signal on line 70 passes through OR gate 288 to set flip-flop 290.The set output of flip-flop 290 is applied to a signal input ofINHIBITED-AND gate 284. Since flip-flop 292 is reset, there is no signalapplied to the inhibit input of lNHlBlTlED-AND gate 284. The signal onoutput line 268 from flip-flop 262 is applied to timer 280 which times ashort duration yellow or clearance interval and then applies an input tothe second signal input of INHIBITED-AND gate 284.

Gate 284 generates a signal which passes through OR gate 286 to triggerone-shot 287 which resets flip-flops 262 and 290 and sets flip-flop 254,returning the controller to its red indication. If another vehicle iswaiting within the detection zone of detector D5, timer 68 times out,and the cycle is repeated to release another vehicle. Thus, so long asthe line of vehicles stretches along entrance ramp 16 to the detectionzone of detector D5, controller 44 cycles through its red-green-yellowsequence at a rate determined by timer 68, timer 270, timer 280, andtime delay circuit 252. The vehicles which reach merging area 20 duringsuch a sequence shorten the line Waiting on entrance ramp 16 to reducethe likelihood of that line interfering with traffic on frontage road18. These vehicles which pass control signal 24 in such a situation canpause in merging area 20 until the driver detects a gap in the trafiicon roadway 10 into which he can merge.

If a car is stopped Within the detection zone of vehicle detector D2 inmerging area 20, either because the driver of the vehicle did not entera gap which was detected by vehicle detector D1 or because a continuingsignal from vehicle detector D5 resulted in the cycling of controller44, an input is applied from detector D2 to timer 60. If the vehiclewaits within the detection zone of detector D2 for the length of timerequired for timer 60 to time out, the timer '60 applies inhibitinginputs to IN- HIBITED-AND gates 53 and 62. These inhibiting inputsprevent the generation of the green indication by controller 44 eventhough an acceptable gap is detected by vehicle detector D1 and eventhough vehicles might be waiting in turning area 22 within the detectionzone of vehicle detector D5. As a consequence, control signal 24 isprevented from releasing a vehicle toward merging area 20 when thatmerging area is already occupied.

Variations in the logic within signal sequence timer and controller 44can of course be made. For example, when the volume of traffic passingvehicle detector D1 is low and the speed is high, the signal applied online 42 to controller 44 can be utilized to reset flip-flops 254, 256,and 262 so that control signal 24 provides no indication at all.Vehicles utilizing entrance ramp 16 will then proceed at the discretionof their drivers. If a particular entrance ramp location does notpresent a possible problem of interference with traific on a frontageroad, such as frontage road 18, then vehicle detector D5, timer 68, IN-HIBITED-AND gate 62, input line 70 to controller 44, AND gate 250, andtime delay 252 can be omitted. Alternatively, in place of traflic speedand volume, other traffic flow parameters such as density or percentageoccupancy can be utilized to control the signal on line 42.

With certain entrance ramps it might be found that the minimum gapduration required for a vehicle to merge onto the limited-access roadwaycan be judged better by utilizing speed of vehicles on thelimited-access roadway at a point immediately upstream of the mergingarea, rather than utilizing the speed of the vehicles passing throughthe detection zone of vehicle detector D2 within the merging area. Insuch a case, an additional vehicle detector, depicted in FIGURE 1 asvehicle detector D6, is provided to monitor the speed of traific in theright-hand lane of the limited-access roadway as it nears the mergingarea, and the input to speed computer 56 is connected to this vehicledetector D6, rather than to vehicle detector D2. Alternatively, anothertraffie characteristic can be used in place of speed to determine therequired gap length. For example, the density or volume of traffic onroadway can be determined by connecting vehicle detector D6 to a densitycomputer or to a volume computer, the output of which is tied to levelmonitor 58. The required gap length is then varied in accordance withtraffic density or trafiic volume on roadway 10. As another alternative,traffic density, volume and speed could be combined to control therequired gap length.

If it is desired to permit only one vehicle at a time to proceed onentrance ramp 16 when a long duration gap exists in the traffic onroadway 10, as indicated by signal G2, then line 54 is omitted. Thencontroller 44 will continuously cycle through its red-green-yellowsequence at a rate determined by timers 270, 276 and 280, as long assignal G1 exists on line 66.

Gap detector 48, depicted in FIGURE 3, can be modified by depletingrelay 134. In this case, relay contact 1321; is coupled to the positivevoltage source through resistor 134g and rheostat 134k and relay contact132:; is coupled to the positive voltage source through resistor 134iand rheostat 134 The output line L4 is also omitted from relay contact1160 in level monitor 58. Then, when the speed of vehicles passingthrough the detection zone of veh detector D2 is in the highest speedrange, no signal is applied from level monitor 58 to gap detector 48.Therefore, relays 128-132 are all de-energized, and charging paths forcapacitors 144 and 164 are provided through resistors 134g and 134i andrheostats 134k and 134i and through all the normally-closed relaycontacts. While the level monitor illustrated in FIGURE 2 and the gapdetector illustrated in FIGURE 3 depict circuit values, these values areonly representative of values which might be utilized in workableembodiments of these circuits and are not limitations.

Level monitor 32 and controlled multivibrator 208 within gap projector50 generate output pulses with a repetition rate dependent upon thelevel of the output voltage from speed computer 30. In place of levelmonitor 32 and controlled multivibrator 208, a voltage controlledoscillator can be utilized.

At some entrance ramps, it might be desired to provide a minimum controlsystem to signal for one car to merge onto the limited-access roadwayeach time a gap of sufficient length occurs in the traffic on theroadway. In such a minimum system, vehicle detector D5, timer 68, gate62, and the associated circuitry Within controller 44, volume computer36, level monitors 34 and 38, AND gate 40 and its associated circuitrywithin controller 44, flipfiop 216, AND gate 218, and the associatedcircuitry within gap projector 50 can be eliminated. Such a minimumsystem will provide a green or go indication each time the signal G1 isgenerated to show that a gap has been detected with a duration longenough to allow a car from entrance ramp 16 to merge into the traffic onroadway 10. A manual control can be provided to turn the system off whenthe trafiic flow does not require its use.

As another alternative, vehicle detector D4 can be eliminated so thateach time signal G1 on line 52 from gap projector 50 indicates that anacceptable gap has reached release point R, controller 44 causes controlsignal 24 to give a green or go indication which continues either untila vehicle passes through the detection zone of vehicle detector D3 oruntil time delay 276 times out to step controller 44 to its yellow orclearance indication. If desired, vehicle detector D3 can also beeliminated so that the duration of the green or go indication fromcontrol signal 24 is terminated only by operation of time delay 276.

While preferred embodiments of particular components of the traificcontrol system have been described, these are illustrative examplesonly, and they are not intended as limitations. Other equivalent devicescan be used in place of one or more of these components and still bewithin the present invention.

The invention has been discussed with reference to a limited-accessroadway and an entrance ramp. It is, of course, obvious that it can beutilized to control traffic entering any heavily travelled main roadwayfrom a com paratively lightly travelled side roadway. Thus, it can beutilized at the intersection of an arterial highway and merging sidestreet or at a merging access road on a bridge or in a tunnel. Inaddition, while traffic in the right-hand lane of roadway 10 has beenmonitored by vehicle detector D1 to determine the presence of acceptable gaps, it would be possible to monitor instead the traffic in adifferent lane or in a combination of lanes on roadway 10.

Level monitor 58 and gap detector 48 have been depicted as dividing thespeed input information into discrete levels and varying the gap timeduration required to generate signal G1 in discrete increments.Obviously a voltage divider network could be utilized to provide smoothvariations instead of incremental variations.

If flip-flop 256 within signal sequence timer and controller 44 requiresit, a one-shot multivibrator can be inserted into the line between theset output of flip-flop 256 and the input of INHIBITED-AND gate 261 toinsure that the pulse which resets flip-flop 256 is of sufficientduration.

Another description of the volume computer suited for use as volumecomputer 36 is found in U.S. Patent No. 2,932,003 issued to John L.Barker, Apr. 5, 1960.

Thus, among others, the several objects of the invention, asspecifically aforenoted, are achieved.

We claim:

1. Apparatus for controlling the entry of vehicles into traffic flow ona main roadway from a side roadway which terminates in a merging areawith said main roadway, said apparatus used with vehicle detection meansof the type which provide indications of vehicle presence and speed andused with signalling means which signals vehicles on said side roadwayapproaching said merging area to enter said main roadway traffic flow,said apparatus comprising in combination:

first electronic circuit means connected to said vehicle detection meansfor determining the occurrence of a gap of at least a pre-set durationin said main roadway traffic flow upstream of said merging area;

a second circuit means connected to said first electronic circuit meansand connected to said signalling means for causing said signalling meansto signal said approaching vehicle to advance to said merging area toallow said approaching vehicle to reach said merging area withsubstantial speed to merge into said gap when said gap is at saidmerging area.

2. Apparatus as claimed in claim 1 further comprising means responsiveto at least one characteristic of traffic flow on said roadways forvarying said pre-set duration.

3. Apparatus for controlling the entry of vehicles into traffic flow ona main roadway from a side roadway which terminates in a merging areawith said main road way, said apparatus used with vehicle detectionmeans of the type which provide indications of vehicle presence andspeed and used with signalling means which signals vehicles on said sideroadway approaching said merging area to enter said main roadway tratficflow, said apparatus comprising in combination:

first electronic circuit means connected to said vehicle detection meansfor determining the occurrence of a gap of at least a pre-set durationin said main roadway trafiic flow upstream of said merging area; asecond electronic circuit means connected to said first electroniccircuit means and connected to said vehicle detection means forpredicting a time when said detected gap is passing a release point onsaid main roadway, said release point being located so that advancementby said approaching vehicle toward said merging area from adjacent saidsignalling means on said side roadway at the time said detected gap ispassing said release point permits said approaching vehicle to reachsaid merging area with substantial speed to merge into said detected gapwhen said detected gap is at said merging area;

third circuit means connected to said second electronic circuit meansand connected to said signalling means for causing said signalling meansto signal to said approaching vehicle to enter said main roadway trafficflow at said predicted point in time.

4. Apparatus as claimed in claim 3 further comprising means connected tosaid third circuit means and connected to said vehicle detection meansfor inhibiting said signalling means from signalling to said approachingvehicle to enter said main roadway traffic flow when another vehicle isstopped within said merging area.

5. Apparatus as claimed in claim 3 further comprising means forrepeatedly causing said signalling means to signal a vehicle to entersaid main roadway traffic flow when vehicles approaching said signallingmeans exist in a line in excess of a pre-set length.

6. Apparatus as claimed in claim 3 wherein said second electroniccircuit means includes means for varying the location of said releasepoint in accordance with a characteristic of trafiic flow on said mainroadway.

7. Apparatus as claimed in claim 3 in which said second electroniccircuit means includes means for simultaneously predicting times wheneach of a plurality of detected gaps is passing said release point.

8. Apparatus as claimed in claim 3 further comprising means responsiveto at least one characteristic of trafiic flow on one of said roadwaysfor varying said pre-set duration.

'9. Apparatus as claimed in claim 8 wherein said responsive means variessaid pre-set duration in accordance with speed on one of said roadways.

10. Apparatus as claimed in claim 8 wherein said responsive meansincludes a timing circuit having a time constant dependent upon saidtrafiic flow characteristics.

11. Apparatus as claimed in claim 8 further comprising means for causingsaid signalling means to continuously signal to approaching vehicles toenter said main roadway traflic flow when said detected gap exceeds asecond pre-set duration.

12. Apparatus as claimed in claim 3 in which said third circuit means isconnected to said vehicle detection means and includes means to enablesaid signalling means to signal to said approaching vehicle to entersaid main roadway trafiic flow at said predicted point in time only itsaid approaching vehicle has reached a point on said side roadwayimmediately upstream of said signalling means.

References Cited UNITED STATES PATENTS 2/1967 Auer 34036 5/1968 Waldron340-36 US. Cl. X.R. 340-31

